Epithelial cells are separated from connective tissue by a basement membrane, which is composed of a variety of extracellular matrix molecules, including proteoglycans, collagen and laminin isoforms. Together these proteins create a framework that is essential for maintaining tissue integrity. However, extracellular matrix proteins play more than just a structural role. They also display a diverse set of biological functions that regulate adhesion, migration, proliferation, differentiation and gene expression of adjacent cells. Roskelly, et al., Curr. Op. Cell Biol. 7:736-747 (1995).
Laminin, of which there are at least ten isoforms, is a major component of basement membranes and has been shown to mediate cell-matrix attachment, gene expression, tyrosine phosphorylation of cellular proteins, and branching morphogenesis. See, e.g. Streuli, et al., J. Cell Biol., 129:591-603 (1993); Malinda and Kleinman, Int. J. Biochem. Cell Biol. 28:957-1959 (1996); Timpl and Brown, Matrix Biol. 14:275-281 (1994); Tryggvason, Curr. Op. Cell Biol 5:877-882 (1993); Stahl, et al., J. Cell Sci. 110:55-63 (1997). The expression patterns of the laminin isoforms are tissue specific. The laminin-5 isoform (nicein, epiligrin, kalinin) is abundant in transitional epithelium, stratified squamous epithelia, lung mucosa and other epithelial glands. Kallunki, et al., J. Cell Biol. 119:679-693 (1992); Jones, et al., unpublished observations. Laminin-5 is a heterotrimer consisting of .alpha.3, .beta.3 and .gamma.2 subunits that associate by means of large .alpha.-helical regions to produce a cruciform-shaped molecule. Rousselle, et al., J. Cell Biol. 114:567-76 (1991); Baker, et al., J. Cell Sci. 109:2509-2520 (1996).
Laminin-5 is synthesized initially as a 460 kD molecule, which undergoes specific processing to a smaller form after being secreted into the extracellular matrix. Marinkovich, et al., J. Biol. Chem., 267:17900-17906 (1992); Matsui, et al., J. Biol. Chem. 270:23496-23503 (1995); Vailly, et al., Eur. J. Biochem. 219:209-218 (1994). The size reduction is a result of processing of the .alpha.3 and .gamma.2 subunits from 200-190 to 160 kD and from 155 to 105 kD, respectively. Marinkovich, et al., J. Biol. Chem., 267:17900-17906 (1992); Matsui, et al., J. Biol. Chem. 270:23496-23503 (1995); Vailly, et al., Eur. J. Biochem. 219:209-218 (1994). The proteases involved in these proteolytic events have not been identified.
Laminin-5 has been reported to function in the nucleation of hemidesmosome assembly and as an adhesive factor that retards cell motility. Baker, et al., J. Cell Sci. 109:2509-2520 (1996); O'Toole, et al., Exp. Cell Res., in press. In contrast, some authors have provided evidence that laminin-5 enhances cell motility and is expressed at the migrating edges of certain tumor cell populations. Zhang and Kramer, Exp. Cell Res. 227:309-333 (1996); Pyke, et al., Am. J. Pathol. 145:782-791 (1994); Pyke, et al., Cancer Res. 55:4132-4139 (1995).
In a number of studies, it has been demonstrated that laminin-5 produced by 804G cells can nucleate the assembly of hemidesmosomes by SCC12, HaCaT and pp126 cells, as well as corneal cells maintained in vitro. Langhofer, et al., J. Cell Sci. 105:753-764 (1993); Hormia, et al., J. Invest. Dermatol. 105:557-561 (1995); Baker, et al., J. Cell Sci. 109:2509-2520 (1996); Baker, et al., Exp. Cell Res. 228:262-270 (1996); Tamura, et al., J. Periodontal. Res., in press. Although SCC12, HaCaT and pp126 cells also secrete laminin-5, the laminin-5 that they secrete is incapable of supporting the assembly of hemidesmosomes.